In the protection of power transmission lines with distance relay elements, which recognize fault conditions on the transmission line, it has always been important to distinguish between various types of faults. Generally, this involves distinguishing between phase-to-ground faults for the three phase lines (A, B and C) of the power signal, and phase-to-phase-to-ground faults involving any two of the phase lines. This is important because of two significant problems in recognizing fault conditions with distance relays.
First, it is necessary to block selected ones of the ground distance elements when there is a phase-phase-ground fault, because one or more of the ground distance elements associated with the actual phase-phase-ground fault can undesirably overreach into an adjoining protected section of the line, which is undesirable because it could result in a false tripping of a circuit breaker which is not responsible for clearing the phase-phase-ground fault. Which particular ground distance element will overreach depends upon the location of the fault resistance, in particular whether the fault resistance is between two phase lines or from a phase pair to ground.
The second problem occurs in certain applications where there is a strong power source behind (upstream of) the relay location. In such a case, a close-in, single-line-to-ground fault in the forward direction can result in one or more of the phase-phase distance elements incorrectly asserting. In certain systems, single pole tripping is desired (where only a particular faulted phase line is interrupted), but the assertion of a phase-phase distance element will result in breakers opening for all three phases. In such a case, an undesired three pole trip thus may occur where only a single pole trip was necessary and desired. This problem is generally referred to as single pole security.
Existing fault identification systems in distance relays typically make use of the phase angle difference between the negative sequence and zero sequence currents for the power signal in their determinations. These systems divide the 360.degree. total phase angle range into three equal sectors, 60.degree.-180.degree., 180.degree.-300.degree. and 300.degree.-60.degree. (through 0.degree.). Different ground and phase-phase elements are blocked from operating, depending upon which of the three sectors the phase angle difference between the negative sequence and zero sequence currents is in. However, it has been found that such systems do not provide satisfactory accuracy, i.e. security, in many situations, particularly as the phase angle difference comes reasonably close to the boundary line between adjacent sectors. Inaccurate fault identification occurs, and wrong distance elements are blocked and distance elements which should be blocked are left unblocked. Hence, a more secure and dependable system for identifying faults in distance relays is desirable.